Inverse Gas Chromatographic Examination of Polymer Composites

Home , Contact angle, Sessile drop technique

Open Chem., 2015; 13: 893–900

Invited Paper Open Access

Adam Voelkel*, Beata Strzemiecka, Kasylda Milczewska, Zuzanna Okulus Inverse Gas Chromatographic Examination of Polymer Composites

DOI: 10.1515/chem-2015-0104 received December 30, 2014; accepted April 1, 2015. of composite components and/or interactions between them, and behavior during technological processes. This paper reviews is the examination of Abstract: Inverse gas chromatographic characterization various polymer-containing systems by inverse gas of resins and resin based abrasive materials, polymer- chromatography. polymer and polymer-filler systems, as well as dental restoratives is reviewed.

Keywords: surface activity, polymer-polymer interactions, 2 Discussion adhesion, dental restoratives, inverse gas chromatography 2.1 Surface energy and adhesion

1 Introduction IGC is useful for surface energy determination of solid polymers and fillers. Solid surface energy of consists of Inverse gas chromatography (IGC) was introduced in 1967 dispersive ( , from van der Waals forces) and specific by Kiselev [1], developed by Smidsrød and Guillet [2], and ( , from acid-base interactions) components: is still being improved. Its popularity is due to its simplicity D and user friendliness. Only a standard gas chromatograph = + (1) is necessary [3] although more sophisticated equipment has been advised. “Inverse” relates to the aim of the experiment. for solids can be calculated according to several It is not separation as in classical GC, but examination of methods [7]; one of the most often used is that of Schultz- the stationary phase properties. Test compounds with Lavielle [8-12]: known properties are injected onto the column containing dd the material to be examined. Retention times and peak RT××ln VN =× 2 N ×× aggsl × + C (2) profiles determine parameters describing the column filling. IGC makes possible polymer and composite study where: at different temperatures and humidities [4-6]. Polymer R = gas constant, 8.314 [J mol-1 K-1]; examination below the glass transition temperature (Tg) T = temperature [K]; 3 allows surface characterization, which can then be used VN = net retention volume [m ]; to monitor surface changes occurring during chemical N = Avogadro’s number, 6.023 × 1023 [1 mol-1]; modification. For liquids, IGC can determine the Flory- a = adsorbate cross sectional area [m2]; -2 Huggins interaction ( and/or ), solubility (δ2), and = surface free energy dispersive component [mJ m ]; three-dimensional Hansen solubility parameters. These = liquid probe surface tension dispersive component allow prediction of polymer mutual solubility, miscibility [mJ m‑2]; C = constant.

Retention data for polar and non-polar test compounds *Corresponding author: Adam Voelkel: Chemical Technology and Engineering, Poznań University of Technology, ul. Berdychowo 4 are necessary to quantify the surface acidic and basic properties ( ) according to the Good-van Oss 60-965 Poznań, Poland, E-mail: [email protected] Beata Strzemiecka, Kasylda Milczewska, Zuzanna Okulus: concept [13]: Institute of Chemical Technology and Engineering, Poznań University of Technology, ul. Berdychowo 4 60-965 Poznań, Poland (3)

are the electron acceptor and donor parameters from difficulty in homogeneously packing the powder sp of the probe molecules. ∆G is the specific component of into tubes [17]. the polar compound’s Gibbs free energy of adsorption. IGC seems a much better method for studying solid Its determination has been described [7-9]. powders, especially for such solids as very hard 1–0.25 mm

Dichloromethane (DM) and ethyl acetate (EA) can be abrasive grains [22]. It is impossible to form a disk and used as test compounds to determine and . DM the Washburn method might be inaccurate. Indirect is a monopolar acid with of 0.0 mJ m-2. Eq. (3) then estimation of the contact angle is impossible due to the reduces to: grains’ irregular shape and small size. IGC made it possible to determine their surface free energy. The authors of this (4) review have also used IGC to characterize carbon black powders [23]. Very often insufficient material is available has been reported to be 5.2 mJ m-2 [14]. Similarly, EA for contact angle measurement. IGC requires only several is a monopolar base and is 0.0 mJ m-2. Thus, for the mg. solid can be calculated: The surface energy of connected solids determines their strength (work) of adhesion, , resulting from (5) dispersive (van der Waals) and acid-base interactions.

is 19.2 mJ m-2 [14]. However, in the literature there are (6) different values for test compound surface free energy components and parameters [15]. Van Oss gave of is the dispersive component and is the acid-base 6.2 mJ m-2 for EA, which differs from that in [16]. Moreover, component. Eq. (6) can be used to determine the work of for dichloromethane (DM) is not given therein, but only adhesion in solid polymer-filler system such as abrasive- that for chloroform (CH; 1.5 mJ m-2) [16]. hardened resin binder [22,24] or polyurethane-carbon We have calculated for the materials black [25]. can be calculated by [26-29]: studied here using from both these sources. The sensitivity of and to the (7) assumed has been discussed [17]. The values calculated from van Oss’ data [16] denote the dispersive components of the filler (f) were three times higher than those calculated according and resin (p) free surface energies. is the component to data from [14]. The values calculated from van due to acid-base interactions:

Oss’ data [16] were from 1/3 to 1/5 those calculated from data in [14]. However, the trends in both parameters were (8) independent of which was used. The were similar as well as the and . Parameters estimated by IGC should not be treated as absolute, but they are very useful are the acidic and basic parameters for materials comparisons. of the polymer surface (p) describing its electron acceptor There are several methods for solid surface energy and electron donor abilities. denote the same determination; contact angle measurement is most often characteristics of the filler (f). used [17]. The use of different polar and nonpolar Eq. (6) does not include the polar component of liquids enables determination of the Lifshitz-van der the work of adhesion due to dipole-dipole and induced Waals ( ) and the Lewis acid-base ( ) components dipole-dipole interactions. Fowkes has demonstrated that of . Moreover, the electron-acceptor ( ) and electron- this contribution is negligible [26,27]. donor ( ) parameters of the acid-base component can be IGC also allows assessment of the Wa/Wcoh ratio for a determined if the components of the liquid surface free filler dispersion in a polymer matrix [24]. Wcoh, the filler energy are known. work of cohesion, is calculated as a sum of dispersive However, this method suffers from some limitations, ( ) and specific ( ) components in the same way as e. g. the diameter of the liquid drop affects the results the work of adhesion [29]: [17] and the solid surface must be smooth [18,19]. The solids are frequently powders and it is impossible to (9) prepare a flat and smooth surface [20]. The Washburn method [21] of powder surface energy estimation suffers (10) Inverse Gas Chromatographic Examination of Polymer Composites 895

If the ratio of Wa/Wcoh is close to 1 the filler cohesion forces polymer, polymer-filler, filler-filler), expressed by the and filler-polymer adhesion forces are in balance. Flory-Huggins parameter [33,34]:

(13) 2.2 Polymer-polymer interactions

Here, the second subscript of Vg identifies the nature of The usefulness of IGC for determining polymer–small the column. molecule interactions is well established. The Flory- To obtain for a polymer blend or composition Huggins interaction parameter is a measure of the utilizing IGC, values for all components must be free energy of interaction between the probe and the known. Therefore, three columns are usually prepared: material tested. Experimental retention parameters (e.g. two containing the single components and the third

Vg – specific retention volume) can be converted to Flory– containing their composite. A further three columns Huggins parameters [30]: containing different composites can also be prepared if the effect of composite proportions is to be examined. These (11) columns should be tested using identical temperature, flow rate, inlet pressure, and test solutes [35]. The usual where: 1 denotes the solute and 2 or 3 denotes polymer sign convention is assumed; i.e., a large positive value o or filler, M1 the molecular weight of the solute, p1 the indicates unfavourable interaction, a low value indicates saturated solute vapour pressure, B11 the solute second favourable interaction, while a negative value indicates a virial coefficient, Vi the molar volume, ρi the density, R the rather strong specific interaction. gas constant, and Vg the specific retention volume. The effects of three variables on the polymer/filler For a filled polymer, Eq. (11) can be rearranged interaction parameter were examined:

[31,32]: –– the type of filler –– the amount of filler –– the test solute. (12)

Although not predicted by the theory, values commonly where φ2 and φ3 are the polymer and filler volume fractions depend on the solute chemical structure [36]. This has and m refers to the composite. been interpreted as due to preferential interactions of the Inverse gas chromatography can also be used to test solute with one of the components. characterize composite component interactions (polymer-

Figure 1: Filler influence on χ23 (PE2 – polyethylene; B – fillers: silica modified with: N-2-aminoethyl-3-aminopropyltrimethoxysilane (B2), 3-aminopropyltriethoxysilane (B3), 3-mercaptopropyl-trimethoxysilane (B4), n-octyltriethoxysilane (B5). 896 Adam Voelkel et al.

Figure 2: Dependence of (chloroform) on the amount of modifier in filler at 363K (I, II, III, IV, V denotes 1, 2, 3, 5, 10% of modifier in filler B2).

Figure 3: Effects of test solutes on 23χ for 10% IIB5+PE.

Milczewska and Voelkel [34,37] discussed some values were obtained if the solvents used satisfied the zero methods of creating a probe-independent interaction ∆χ criterion. parameter. Zhao and Choi [31,32] proposed a ‘common Zhao and Choi proposed two equations for : reference volume’ (Vo, the smallest polymer repeat unit molar volume) to remove the problem. Their definition (14) of χ differed from the traditional definition by the ratio of reference to probe volume (Vo/V1). When a common and reference volume was used, the data conformed to the ternary Flory-Huggins lattice theory and unique (15) Inverse Gas Chromatographic Examination of Polymer Composites 897

Figure 4: Values of calculated by the Zhao-Choi procedure for Aerosil®200V-PLA compositions (numbers correspond to component weight fractions).

Eq. (15) predicts that a plot of versus by light as in conventional composites [41,48]. Compomers ( ) will give a straight line with slope 1 and are polyacid-modified composites, in which setting occurs intercept (- ). first by photopolymerization and subsequently by a slow acid-base reaction after water sorption [41,49]. All should adhere well to tooth tissues and prevent 2.2 Activity of tooth tissue and dental microorganism adhesion. Both behaviors crucially restorative depend on the surface properties [50]. Dental caries starts with a bacterial biofilm (dental plaque) covering Another interesting application of inverse gas the tooth surface [51-53]. High surface energy contributes chromatography is the examination of polymeric dental to this phenomenon. is the most important parameter restorative materials’ surface properties. Preceded by the because low values significantly reduce bacterial ability removal of infected tissue, their application is the only to adhere in comparison with materials with high way to prevent dental caries – which affects people all [54-55]. over the world [38]. Caries consists of tooth destruction Rüttermann et al. investigated dental composite by demineralization of the inorganic part and destruction surface energy [54-55] by the contact angle method. This of the organic one by microorganisms [39]. Replacement method is most popular but a false result can be obtained restores tissue continuity and function. if the surface is not smooth [18], and it is very important to Polymers are used in all fields of dentistry [40]. use the same drop diameter [56]. Polymeric restoratives include a very wide spectrum of Inverse gas chromatography avoids these problems types, including composites, glass-ionomer cements, and has been used to estimate materials’ susceptibility resin-modified glass-ionomer cements and compomers to biofilm formation. Both dispersive [57-58] and specific [41]. All have very complex compositions; their properties [59] surface energy components have been determined reflect this complexity. Composites consist mainly of an for glass-ionomer cements, which are known for their organic matrix and inorganic filler in varying proportion. ability to absorb water. These show highest surface In many cases a coupling agent to improve their adhesion activity (strongest dispersive interactions) at lowest is required. Polymerization initiators allow hardening humidity [57-58] and increased wet storage time increases by blue light photopolymerization [41-47]. Glass- their surface activity [58]. The surface is strongly basic ionomer cements are composed of polycarboxylic acid, and acid-base interactions decrease at higher humidity fluoroaluminosilicate glass, water, and tartaric acid. [59]. This information is important in applications. This Setting between filler and matrix occurs by acid-base suggests that IGC will also be successful in examination of reaction [41,48]. Resin-modified glass-ionomer cements other, potentially more stable materials applied as dental contain methacrylate resin. This allows a dual curing fillings. method – the first, a conventional glass-ionomer acid-base The surface energy of bovine tooth tissues was reaction starts upon mixing, while the second is initiated also examined by means of IGC. Tissue type (dentin or 898 Adam Voelkel et al.

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